Parasitic protozoan infections are a major concern for human health. Toxoplasmosis is a parasitic infection caused by Toxoplasma gondii (T. gondii). Although toxoplasmosis is most often asymptomatic, persons infected with toxoplasmosis can experience severe symptoms, including seizures, poor coordination, lung damage, eye damage, and brain damage, and the infection in immunocompromised patients is often fatal if not treated. Other parasitic protozoan infections include leishmaniasis (also known as leishmaniosis), caused by protozoans of genus Leishmania, including Leishmania major (L. major) Leishmania tropica (L. tropica), Leishmania brasihensis (L. brasiliensis), and Leishmania donovani (L. donovani); Chagas disease, caused by the protozoan Trypanosoma cruzi (T. cruzi); Human African Trypanosomiasis (also known as HAT and African sleeping sickness), caused by the protozoan Trypanosoma brucei (T. brucei); and Malaria, caused by protozoans of genus Plasmodium, including Plasmodium falciparum (P. falciparum).
Existing treatment for toxoplasmosis include administration of pyrimethamine, usually in combination with a DHPS sulfonamide inhibitor (e.g., sulfadiazine) to improve efficacy and leucovorin to improve tolerability. Allergic reactions to sulfonamide drugs are common and therefore some patients are not able to receive the combination therapy. Pyrimethamine treatment may cause severe side-effects and toxicity, including nausea, vomiting, leukopenia, bone marrow toxicity, teratogenicity and central nervous system toxicity. Mechanism-based toxicity of DHFR inhibition in mammalian, including human, cells can be partially alleviated by administration of leucovorin to selectively replace tetrahydrofolate in mammalian cells.
Pyrimethamine acts by inhibiting the enzyme dihydrofolate reductase (DHFR). The IC50 for pyrimethamine against T. gondii DHFR (tgDHFR) is 0.76 μM, while that against human DHFR (hDHFR) is 5.8 μM. (Allegra et al., J. Clin. Investigation. 1987, 79, 478-482.) Thus, although pyrimethamine inhibits tgDHFR more potently than hDHFR, the selectivity ratio for tgDHFR—less than 10—is relatively low. Therefore, clinically relevant doses of pyrimethamine result in plasma concentrations that effectively inhibit hDHFR, leading to many of the observed mechanism-based side effects of pyrimethamine. Furthermore, the relatively high IC50 for pyrimethamine against tgDHFR requires greater concentrations in plasma for efficacy, which may cause additional, off-target induced side effects.
Thus, there is a need for compounds that are both more potent inhibitors of tgDHFR and more selective inhibitors of tgDHFR over hDHFR. Likewise, there is also a need for potent and selective DHFR inhibitors against Leishmania, T. cruzi, T. brucei and Plasmodium for the treatment of leishmaniasis, Chagas disease, African Trypanosomiasis, and Malaria, respectively.